50165-597, Issue 3
Model 750 Audio Mixer
User Guide
Issue 3, May 1997
This User Guide is applicable for serial numbers 201 and later.
© 1997 by Studio Technologies, Inc., all rights reserved
5520 West Touhy Avenue
Skokie, Illinois 60077 U.S.A.
Telephone (847) 676-9177
Fax (847) 982-0747
www.studio-tech.com
Model 750
Model 750 User Guide Issue 3, May 1997
Studio Technologies, Inc. Page 3
Model 750
Table of Contents
Foreword
Introduction .................................................................. 7
System Features.......................................................... 8
Installation .................................................................... 10
Configuration ............................................................... 16
Operation ..................................................................... 23
Technical Notes ........................................................... 36
Specifications............................................................... 38
Appendix A-1 ............................................................... 40
Appendix A-2 ............................................................... 41
Block Diagrams
Model 750 User Guide Issue 3, May 1997
Studio Technologies, Inc. Page 5
Model 750
Foreword
With all the pressures in life to get things done faster, work more hours, finish one job so
you can get on to the next, its rare to get a chance to try and do something really right.
Thats what Studio Technologies attempted to do with the Model 750 Audio Mixer. The
goal was simple, to make a great audio mixer for ENG, SNG, and mobile broadcast appli-
cations. By great we meant not just meeting specifications or price issues, but to address
the real needs of personnel out in the field. And not only to meet their immediate needs,
but to provide resources that they can adopt and use in the future. While getting some-
thing perfect is impossible, I can say that I'm very pleased with how the product turned
out, and hope that you share my enthusiasm. I think that you'll find the Model 750 a plea-
sure to use, providing the resources required for efficient, easy-to-use, day-after-day
operation.
A special thanks to Tony Raven, Chief Engineer at Shook Electronics USA. It was Tony
who suggested that Studio Technologies create an audio mixer for mobile broadcast
applications. He supported his idea by spending many hours with me on the telephone,
reviewing lists of features, making comments, and keeping us focused on meeting the
needs of field personnel. Thanks again Tony!
Many other broadcast professionals contributed their time, expertise, and good ideas to
help make the Model 750 what it is. Without their help the unit would definitely not be as
finely tuned as it is. Thanks to you all!
Many people at Studio Technologies contributed toward making the Model 750 a reality.
Fred Roeck did the mechanical design and prepared the production documentation.
Carrie Loving designed the graphics and provided engineering CAD support. Al Lux de-
signed the printed circuit boards. Mitch Budniak, Ace Consulting Engineer, designed the
power supply and did several overall reality reviews. Jim Cunningham, with assistance
from Gary Hebert at THAT Corporation, designed the limiter/compressor circuit. Joe
Urbanczyk designed the computerized test routines for the Audio Precision System One
and worked on agency approvals. Larry Leviton wrote the software that drives the itty-bitty
micro-controller in the Setup section800 bytes in C! Minas Serbekian and his crew built
the prototype units, ensuring that the final design was "manufacturable."
Please contact me with your questions, comments, and suggestions. I can be reached by
voice at (847) 676-9177, fax at (847) 982-0747, or via the Internet @ www.studio-tech.com.
Sincerely,
Gordon Kapes
President
Model 750 User Guide Issue 3, May 1997
Studio Technologies, Inc. Page 7
Model 750
Model 750
Introduction
The unique requirements of electronic
news-gathering (ENG) trucks, satellite
news-gathering (SNG) trucks, and special-
ized remote broadcast applications
demand more than just a general-purpose
audio mixer. Thats why Studio Technolo-
gies created the Model 750, a special
piece of equipment for a special applica-
tion. Only by taking the time to speak with
industry professionalsdesigners of ENG
and SNG vehicles, along with numerous
field technical personnelwere the neces-
sary specifications established. Using that
information as a road map to guide the
process, additional features, operating
ergonomics, and overall look-and-feel
Figure 1. View of Model 750 Front Panel
Figure 2. View of Model 750 Back Panel
issues were addressed. The result is a
high-performance, easy-to-use audio
mixer expressly designed to go on-the-
road.
What This User Guide Covers
This User Guide is designed to assist you
when installing, configuring, and using the
Model 750 Audio Mixer. A limited amount
of background technical information is
also provided. Should you require detailed
technical information please refer to the
Model 750 Service Guide. The Service
Guide contains detailed service informa-
tion, including schematic diagrams. The
Service Guide is not shipped with each
Model 750, but is available from the fac-
tory upon request.
Issue 3, May 1997 Model 750 User Guide
Page 8 Studio Technologies, Inc.
Model 750
System Features
Mic/Line Inputs
Four transformer-coupled input channels
are provided for connection to micro-
phone or line-level signals. The circuitry is
ruggedized for reliability under tough
operating conditions. A full set of features
is provided for each input channel, includ-
ing selectable input sensitivity, +48V
phantom power, low-cut filter, phase
reverse, level control, and pan pot. For
operator assistance, a bi-color LED pro-
vides signal present and peak level indica-
tion. Two illuminated push-button switches
control the routing of the input signal to
the main and auxiliary (AUX) audio buses.
For convenience the four mic/line input
sensitivity buttons, like all the Model 750s
operator controls, are located on the front
panel.
Stereo Line Inputs
Two stereo line-level input channels are
provided, and are intended for connection
with audio signals associated with video
playback. The channels support real
world broadcast applications, where left
and right signals may, or may not, be an
actual stereo pair. Routing buttons allow
each left and right input to be assigned to
the left output bus, to the right output bus,
to both output buses, or muted. A level
control, along with a bi-color signal
present/peak LED, supports each left
and right input. Two illuminated buttons
control the routing of the signals to the
main and AUX audio buses.
Main Output
Signals from the mic/line input channels,
stereo line input channels, and the setup
section (discussed later) combine to
create the main audio output. The main
output bus is stereo, but can be used to
create two independent monaural audio
mixes. Two studio-quality, stereo limiter/
compressor circuits are provided to con-
trol the dynamic range of the main audio
bus. Far from a simple clipper, each
limiter/compressor circuit utilizes a sophis-
ticated laser-trimmed voltage-controlled
amplifier (VCA) integrated circuit for quiet,
low-distortion operation. For installation
flexibility, three sets of stereo output cir-
cuitry provide access to the main stereo
bus: two are electronically balanced, line-
level; the third is transformer-balanced,
with its output level selectable for mic or
line.
AUX Output
The Model 750 provides a second stereo
audio bus that can be configured to oper-
ate in either of two modes: as an auxiliary
(AUX) output or as a monitor interrupt
function. In the AUX output mode special
applications are easily supported, such as
creating a mix-minus feed or serving as
a second stereo output for on-air use.
Signals from the mic/line input channels,
stereo line input channels, and the setup
section can be assigned to the AUX bus.
Access to the AUX bus is provided by
means of an electronically-balanced, line-
level output.
In most cases, however, the AUX interrupt
mode will be selected, allowing the normal
monitor source to be interrupted and the
AUX signal monitored in its place. Model
750 operators will find this function ex-
tremely useful, enabling one or more of
the input channels, or the setup section,
to be monitored without interfering with
signals assigned to the main output bus.
Two DIP switches, accessible from the
Model 750 User Guide Issue 3, May 1997
Studio Technologies, Inc. Page 9
Model 750
front panel, allow the interrupt function to
be configured; one switch selects whether
the monitor speaker output will be inter-
rupted by the AUX signal, the other
whether the headphone output will be
interrupted.
Configurable Outputs
Four monaural, balanced, line-level out-
puts can serve in a wide range of installa-
tion-specific applications. DIP switches,
located on the back panel, are used to
select from the five available signal
sources: main output left, main output
right, AUX output left, AUX output right,
and mic/line input channel direct. The DIP
switches can be set to provide additional
stereo outputs, create monaural outputs
from the main or AUX outputs, or to pro-
vide direct outputs from the mic/line input
channels.
Monitor Section
Reminiscent of much larger audio con-
soles, the Model 750s monitor section
gives the operator an extensive set of
resources. Separate stereo outputs and
level controls are provided for connection
to a monitor amplifier and headphones. In
addition to monitoring the main and AUX
audio buses, provision has been made for
monitoring two external audio sources.
These stereo inputs are intended to be
connected to off-air, microwave, or satel-
lite receivers. External Monitor Input 1 is
compatible with +4 signals, while Exter-
nal Monitor Input 2 can be set, using a
front-panel DIP switch, for 10 or +4
compatibility.
Multiple sources can be simultaneously
selected for monitoring. This can prove
useful by allowing, for example, an opera-
tor to simultaneously monitor a local mix
and an externally-provided mix-minus
signal. A mono function allows the moni-
tored audio source to be checked for
phase cancellation problems and accept-
able sum-to-monaural performance.
The AUX interrupt mode allows monitoring
of selected input channels and the setup
section without interfering with normal
operation of the main audio bus. For
operator assistance, the meters can be
configured to monitor the AUX bus when-
ever the AUX interrupt mode is active.
Mechanical VU-type meters, along with
peak level LED indicators, provide a user-
friendly indication of audio-signal levels.
Fanatics for addressing the details, Studio
Technologies has even eliminated the
issue of burned-out meter-scale illumina-
tion by implementing a solid-state, LED-
based lighting scheme.
Setup Section
Unique to the Model 750 are the features
provided by the setup section, the high-
light being the ability of the operator to
record and playback three voice identifica-
tion segments. The Left+Right segment
allows a voice message of up to 12 sec-
onds in length to be recorded, with play-
back going to the left and right output
channels. The Left Only and Right Only
segments allow recording of up to 4 sec-
onds each, with playback going only to
their respective channels.
The voice segments can prove useful,
allowing the facilities that receive the
vehicles signal (generally provided by
microwave or satellite links) to quickly
learn more about the origin of the trans-
mission. Information such as vehicle
identity, physical location, the names of
technical and on-air personnel, mobile
Issue 3, May 1997 Model 750 User Guide
Page 10 Studio Technologies, Inc.
Model 750
telephone numbers, and left and right
channel identifiers can easily be sent.
The voice audio source is selectable, with
the choices being either the internal micro-
phone or an externally-connected signal.
The setup section also contains a digitally-
controlled sine-wave generator which,
under software control, produces a
0-level single-tone reference and multi-
step tone sequences. The single-tone
output is selectable for either 400Hz or
1kHzthe two industry-standard frequen-
cies. The tone sequences allow frequency
response testing of a transmission chain,
with the choice of either a basic or an
advanced sequence. In the basic setting
a 1kHz 0-level reference is generated,
followed by individual 10kHz, 1kHz, and
100Hz tones. In the advanced setting a
1kHz reference is generated, followed by
14 individual tones, ranging from 15kHz to
50Hz. At the end of each sequence a left/
right channel identifier is generated. It
consists of a short burst of 700Hz sent
only to the left channel, followed by two
short bursts of 700Hz to the right channel.
While the voice and tone setup signals can
be assigned to the main and AUX output
buses, a direct line-level stereo output is
also provided. This allows the generation
of setup and test signals without interfer-
ing with normal operation of the main and
AUX audio buses.
Talk Back
The Model 750 includes a talk back
function. Two push-button switches allow
the selected voice source to be routed to
the setup direct output and/or the main
and AUX outputs.
Mounting, Connectors, and Mains
While the Model 750 is loaded with fea-
tures, it only requires two spaces in a
standard 19-inch rack. XLR-type and
¼-inch 3-conductor audio connectors are
used for audio interconnection. Mains
power is factory configured for 100, 120,
or 220/240V, 50/60Hz.
Installation
In this section you will be installing the
Model 750 Audio Mixer in an equipment
rack. Input and output connections will
be made using the Model 750s multitude
of jacks. AC mains power will then be
connected.
During normal operation, no access to the
back panel of the Model 750 is required.
However, there are five trim potentiom-
eters that may need to be adjusted prior to
the Model 750 going into service, depend-
ing on the requirements of the specific
installation. Since part of the calibration
process involves observing the front-
panel-mounted VU meters, you may want
to perform the calibration process on-the-
bench, prior to mounting the Model 750
in an equipment rack. In addition, there
are a number if DIP switches that may
need to be set. Refer to the Configuration
section of this guide for details.
Realistically, as long as you can get the
assistance of another person for about
5 minutes, go ahead and install the Model
750, wire it up, and then worry about the
calibration. You will need a highly-skilled
and articulate helper, capable of describ-
ing the position of the VU meters indica-
tors with words such as: too high,
down a little, almost, up a tad,
and OKyouve got it!
Model 750 User Guide Issue 3, May 1997
Studio Technologies, Inc. Page 11
Model 750
Locating the Unit
Physical access and mechanical noise
are the primary factors when choosing a
mounting location. By its very nature as an
audio mixer, an operator will need easy
access to all the controls, as well as good
sight lines to the meters. In most cases the
internal microphone, located on the upper
right side of the front panel, will be used,
so be careful that you dont have a cooling
fan blowing directly onto it.
As a device that contains high gain audio
stages, hum and noise pickup is possible
through the chassis and associated ca-
bling. Locating the unit away from devices
that radiate strong AC fields is an excellent
idea. The left side of the Model 750, which
contains the microphone preamplifiers, is
the area most sensitive to noise pickup.
Also, it is possible that the Model 750s
power transformer may radiate energy into
adjacent equipment. It is located at the
right, bottom rear corner of the unit, and
may need to be physically isolated from
sensitive equipment, especially videotape
recorders.
Mounting
The Model 750 is intended for rack mount-
ing, requiring two standard rack spaces
(3.5 inches). It weights approximately 14
pounds (6.4kg), and operates on either
100, 120, or 200/240V, 50/60Hz. The unit is
secured to the front mounting rails of an
equipment rack using two mounting
screws per side.
Audio Inputs and Outputs
The Model 750s audio input and output
connections are made using 3-pin XLR-
type and ¼-inch 3-conductor phone jacks.
Balanced Connections
(Input & Output)
Sleeve
(Shield)
(Switchcraft No. 297, Neutrik NP3C, or equivalent)
Tip ( + )
Ring ()
Dont be concerned about our use of
phone jacks as part of a professional
product. The jacks used feature gold-
plated contacts for high reliability. As
much as we wanted to, there was simply
no room in only two rack spaces to use all
XLR-type connectors.
Caution: For reliable audio intercon-
nection, the phone plugs you use must
comply with industry-standard RS-453.
Switchcraft No. 297 or Neutrik NP3C
are highly recommended.
Mic/Line Inputs
The Model 750 contains four input chan-
nels that are compatible with microphone
or line-level sources. Each of the four
Mic/Line inputs use a single 3-pin female
XLR-type connector. Prepare the mating
connectors (males) so that pin 2 is signal
high (+ or hot), pin 3 is low ( or cold),
and pin 1 is shield.
It is expected that the mic/line inputs will
be wired to input/output connector panels
located on the side or rear of the vehicle.
Its also nice to have one or two of the
inputs multed to connectors inside the
vehicle, allowing the quick connection of
Figure 3. Recommended phone plugs
Issue 3, May 1997 Model 750 User Guide
Page 12 Studio Technologies, Inc.
Model 750
electronically balanced, have a nominal
level of +4dBu, and are capable of driving
balanced or unbalanced loads of 600
ohms or greater. Main Output 3 is trans-
former balanced, with its output level
selectable, using a front-panel button, for
microphone or line level. It is capable of
driving full level into balanced or unbal-
anced loads of 2k ohms or greater.
Main Output 1 and 2
Main Output 1 and 2 are intended for
connection to devices located within the
vehicle. It is recommended that either
Main Output 1 or 2 be used as the on-air
source. Each output utilizes two ¼-inch
3-conductor phone jacks for interconnec-
tion. Prepare the mating connectors
(plugs) so that tip is signal high (+ or hot),
ring is low ( or cold), and sleeve is shield.
Whether these outputs are wired via the
patch bay will depend on the specific
installation.
Note that while the electronically-balanced
output circuitry is capable of driving loads
of 600 ohms or greater, the output level
will drop slightly as the load impedance
approaches 600 ohms. A 0.5dB difference
in output level can be expected as the
load impedance changes from 10k ohms
to 600 ohms.
While balanced operation is preferred,
unbalanced operation is not a problem
for the electronically-balanced output
circuitry. To connect to an unbalanced
load connect the tip to high (+ or hot),
and both the ring and sleeve to shield. For
optimal unbalanced operation, it is impor-
tant to connect both the ring and sleeve
together on the plug that mates with the
Model 750.
microphone or line-level signals. To mini-
mize noise and other problems, it is rec-
ommended that the mic/line signals arent
wired by way of points on the patch bay.
Stereo Line Inputs
The Model 750 contains two stereo line-
level input channels. They are intended
for connection to sources located within
the vehicle, specifically the audio output
channels from videotape machines. The
direct-coupled, electronically-balanced
circuitry has extremely low noise, high
common-mode rejection, but no special
protection from overvoltage or static.
(Dont get us wrongthe stereo line input
circuitry is better than whats found on
most audio mixers available today
theyre just not as tough as the mic/line
inputs!) The stereo line input circuitry is
intended for connection to balanced, line-
level signals with a nominal operating level
of +4dBu.
Each stereo line input channel uses two
3-pin female XLR-type connectors. Pre-
pare the mating connectors (males) so
that pin 2 is signal high (+ or hot), pin 3 is
low ( or cold), and pin 1 is shield. With an
unbalanced source connect pin 2 to high
(+ or hot), and pins 1 and 3 to shield. If
connecting to an unbalanced source in
this manner results in hum or noise, con-
nect pin 2 to high (+ or hot) and pin 3 to
shield; leave pin 1 unterminated. For
flexibility of use it is recommended that the
stereo line input connections be routed
through points on the patch bay.
Main Outputs
The Model 750 contains three sets of
stereo outputs associated with the main
audio bus. Main Outputs 1 and 2 are
Model 750 User Guide Issue 3, May 1997
Studio Technologies, Inc. Page 13
Model 750
Even if there are no immediate plans
to connect the AUX audio outputs to
external devices, it is recommended that
they be terminated on patch points. In this
manner they will be accessible for those
one-in-a-million situations that seem to
arise far too frequently!
Configurable Outputs
Each of the four configurable outputs is
electronically balanced, has a nominal
level of +4dBu, and is capable of driving
balanced or unbalanced loads of 600
ohms or greater. The outputs are intended
to be used only within the vehicle. DIP
switches are associated with each
configurable output. Details on using
these switches can be found in the Con-
figuration section of this guide.
Each of the configurable outputs is acces-
sible via a ¼-inch 3-conductor phone jack.
Prepare the mating connectors (plugs) so
that tip is signal high (+ or hot), ring is low
( or cold), and sleeve is shield. To con-
nect to an unbalanced load connect the
tip to high (+ or hot), and both the ring
and sleeve to shield.
It is highly recommended that the config-
urable outputs be routed via the patch
bay. Or, if there are no immediate needs
for these outputs, be certain to terminate
them on the patch bay for future use.
Sometime, somewhere, they will get used.
Setup Direct Output
Direct access to the setup sections voice
and tone signals is provided by means of
a stereo line-level output. As one of the
unique features of the Model 750, it should
definitely be utilized! This output is in-
tended to be connected to the audio input
associated with the video/audio switchers
Main Output 3
Main Output 3 has been optimized for
connection to devices located external
to the vehicle. This ruggedized, trans-
former-coupled stereo output is select-
able, using a front-panel button, for
microphone or line level. It is recom-
mended that Main Output 3 be wired, via
points on the patch bay, to the vehicles
input/output connector panel. Two 3-pin
male XLR-type connectors are used for
interconnection. Prepare the mating con-
nectors (females) so that pin 2 is high
(+ or hot), pin 3 is low ( or cold), and pin
1 is shield.
Main Output 3 was specifically designed
to provide a fully-isolated output with good
sonic performance. It was NOT designed
to drive full level into 600 ohm loads! While
a 600 ohm load can be connected without
damage, a 2dB level drop should be
expected. Used with 2k ohm or greater
loads, Main Output 3 will provide its full
specified performance.
AUX Output
A stereo output is provided to give access
to the Model 750s AUX audio bus. It is
intended that the AUX audio output will be
connected to devices within the vehicle.
The electronically-balanced, line-level
outputs have a nominal level of +4dBu,
and are capable of driving balanced or
unbalanced loads of 600 ohms or greater.
Two ¼-inch 3-conductor phone jacks
used for interconnection. Prepare the
mating connectors (plugs) so that tip
is signal high (+ or hot), ring is low ( or
cold), and sleeve is shield. To connect to
an unbalanced load connect the tip to
high (+ or hot), and both the ring and
sleeve to shield.
Issue 3, May 1997 Model 750 User Guide
Page 14 Studio Technologies, Inc.
Model 750
bars and tone position. For maximum
flexibility, it is recommended that these
signals be connected by way of the patch
bay. In this way they can be accessed for
test and calibration purposes.
The electronically-balanced, nominal
+4dBu outputs are accessible via two
¼-inch 3-conductor phone jacks. The
circuitry is capable of driving balanced or
unbalanced loads of 600 ohms or greater.
Prepare the mating connectors (plugs) so
that tip is signal high (+ or hot), ring is low
( or cold), and sleeve is shield. To con-
nect to an unbalanced load connect the tip
to high (+ or hot), and both the ring and
sleeve to shield.
Two trim potentiometers are used to set
the output level of the setup direct output.
Refer to the Configuration section of this
guide for details.
External Monitor Inputs
Two external audio signals can be con-
nected, allowing them to be monitored
independently of the internal audio buses.
It is intended that the external monitor
inputs will be connected to outputs associ-
ated with off-air, microwave, or satellite
receivers. Both inputs are stereo, elec-
tronically balanced, and interface using
¼-inch 3-conductor phone jacks. External
Monitor Input 1 is compatible with signals
that have a nominal level of +4dBu. Exter-
nal Monitor Input 2 is compatible with
10dBu or +4dBu signals. A DIP switch,
located on the front panel of the Model
750, is used to set the input sensitivity.
Refer to the Configuration section of this
guide for configuration details.
As an installation aid, a connection made
only to the jack labeled L/MONO routes
the signal to both the left and right input
circuitry. This allows a monaural signal to
be monitored in a 2-channel mono format.
If you wish to have a mono input con-
nected only to the left input, simply insert
an unterminated plug into the associated
right input jack. This will break the nor-
mal connections that link the left and right
input circuits.
Prepare the mating connectors (plugs)
so that tip is signal high (+ or hot), ring is
low ( or cold), and sleeve is shield. With
an unbalanced source connect the tip to
high (+ or hot), and both the ring and
sleeve to shield. If connecting to an unbal-
anced source in this manner results in
hum or noise, connect tip to high (+ or
hot) and ring to shield; leave the sleeve
unterminated.
Monitor Output
The monitor output is designed for con-
nection to an audio power amplifier asso-
ciated with a pair of monitor loudspeakers.
In mobile applications it may prove com-
mon for loudspeakers with internal amplifi-
ers, such as those from Fostex, to be
connected to the Model 750s speaker
output. The outputs are electronically-
balanced, line-level, and can drive bal-
anced or unbalanced loads of 600 ohms
or greater.
The monitor output utilizes two ¼-inch
3-conductor phone jacks for interconnec-
tion. Prepare the mating connectors
(plugs) so that tip is signal high (+ or hot),
ring is low ( or cold), and sleeve is shield.
To connect to an unbalanced load con-
nect the tip to high (+ or hot), and both
the ring and sleeve to shield. Whether
these outputs are wired via the patch bay
will depend on the specific installation.
Model 750 User Guide Issue 3, May 1997
Studio Technologies, Inc. Page 15
Model 750
Record/Talk Back External
Audio Input
The Model 750 contains an internal micro-
phone which is used by the setup section
for voice recording, as well as by the talk
back functions. In special cases it may be
desirable for an external source of audio
to be used in its place. A ¼-inch 3-con-
ductor phone jack allows the connection
of a line-level signal, with a nominal level
range of 20 to +4dBu.
Prepare the mating connector (plug) so
that tip is signal high (+ or hot), ring is low
( or cold), and sleeve is shield. With an
unbalanced source connect the tip to high
(+ or hot), and both the ring and sleeve
to shield. If connecting to an unbalanced
source in this manner results in hum or
noise, connect tip to high (+ or hot)
and ring to shield; leave the sleeve
unterminated.
It is intended that a microphone associ-
ated with an intercom system headset will
serve as the external audio source. Inter-
com stations often provide an amplified,
line-level version of the mic signal. It is
often referred to as a hot microphone
output.
A trim potentiometer is associated with the
external audio input, allowing adjustment
of the input sensitivity. Refer to the Con-
figuration section of this guide for details.
Information is also provided on how to set
the front panel DIP switch to select the
external audio input.
Connecting AC Mains Power
The Model 750 is factory configured to
operate from AC mains power of either
100, 120, or 220/240V, 50/60Hz. In most
cases, units shipped to North America
are factory configured for 120V operation
and are supplied with an internal 0.5A,
5 x 20mm fast-acting fuse. Units config-
ured for 100V also utilize a 0.5A fuse. Units
configured for 220/240V operation are
supplied with a 0.25A, 5 x 20mm fast-
acting fuse.
Before connecting the Model 750 to mains
power, determine the actual mains voltage
and confirm that the Model 750 has been
configured correctly. As could be ex-
pected, an incorrect mains configuration
could seriously damage the unit.
Should it be necessary to change the
units operating voltage refer to the Ser-
vice Guide, a separate document available
upon request from Studio Technologies.
Figure 4. Detail of back panel showing voltage
configuration chart
Issue 3, May 1997 Model 750 User Guide
Page 16 Studio Technologies, Inc.
Model 750
The Model 750 uses an IEC standard
connector to mate with the AC mains cord.
Normally the cord supplied has a North-
American standard plug on one end and
an IEC connector on the other. In non-
North American applications the appropri-
ate plug must be attached. The wire colors
in the mains cord will, in most cases,
conform to the internationally recognized
CEE color code and should be wired
accordingly:
Connection Wire Color
Neutral (N) Light Blue
Live (L) Brown
Earth/Ground (E) Green/Yellow
Because the Model 750 contains no power
switch it will start operating as soon as AC
mains power is connected.
Configuration
As in life, power leads to responsibility.
The Model 750 is no exception as its
features, flexibility, and operational power
leads to a number of configuration issues
that must be reviewed once installation
has been completed. All are quite simple
and, with the exception of the front panel
configuration DIP switches, will only need
to be addressed once.
Configurable Outputs
The Model 750 contains four configurable
outputs, the likes of which arent found on
other audio mixers. Their uniqueness may
make them seem a bit confusing at first
glance. However, they arent too weird,
simply being electronically-balanced
output circuits whose input source is
selectable, rather than being fixed. The
hardest part of using the configurable
outputs is deciding which audio source, or
sources, should be assigned to which
output! By carefully reviewing the choices,
and how their selection may benefit an
installation, the power of this humble out-
put should become apparent. These four
outputs may well eliminate the need for
external distribution amplifiers, combining
amplifiers, and microphone preamplifiers.
Figure 5. Detail of back panel showing
configurable output section (typical of 4)
Model 750 User Guide Issue 3, May 1997
Studio Technologies, Inc. Page 17
Model 750
The five choices are main output left, main
output right, AUX output left, AUX output
right, and mic/line direct:
Main output left and main output right
are the same signals that connect to the
output circuitry associated with the main
outputs.
AUX output left and AUX output right
are the same signals that connect to the
output circuitry associated with the AUX
line-level output.
Mic/line direct is a signal that is derived
from its associated mic/line input chan-
nel. The signal is the same as that
connected to the pan-pot; post input
preamplifier, low-cut filter, and phase
reverse function. Configurable output 1
has access to the signal from mic/line
input 1; configurable output 2 has
access to the signal from mic/line input
2; etc.
Using the five DIP switches associated
with each configurable output, simply
select the source or sources desired. Note
the word sourcesmore than one
source can be selected! As an example,
selecting switch 3, AUX out left, and switch
4, AUX out right, will produce a monaural
(L+R) version of the AUX output. Selecting
switch 1, main out left, for configurable
output 1, and switch 2, main out right, for
configurable output 2 will result in another
stereo output, viola, main output 4!
It would be the authors choice to use
the configurable outputs in this way:
Configurable output 1 set for a mono
version of the main output, configurable
output 2 set for a mono version of AUX
output, and configurable outputs 3 and 4
set for mic/line direct outputs. Terminating
on the patch bay, these four outputs could
provide quite handy for those once in a
blue moon situations that seem to occur
about once a month!
The two mic/line direct outputs may prove
especially helpful, providing two channels
of mic/line amplifier for general purpose
or emergency use. In special situations, a
signal of virtually any audio level could be
bumped to nominal +4dBu.
Figure 6. Detail of back panel showing level
adjustment trim pots for setup direct output
Issue 3, May 1997 Model 750 User Guide
Page 18 Studio Technologies, Inc.
Model 750
Setup Direct Output
The setup direct output is a stereo line-
level output that provides direct access
to the voice and tone signals. Trim potenti-
ometers allow adjustment of the output
level from approximately 2 to +10dBu.
The trim pots are located on the back
panel, directly above the setup direct
outputs ¼-inch phone jacks. At the fac-
tory these outputs are loaded with 100k
ohms, and then the trim pots are adjusted
to give an output level of +4dBu.
If the setup direct output is to be used in
an installation, its important that the left
and right channel output levels be ad-
justed to precisely match the requirements
of the system. Even if you simply want to
use the standard output level of +4dBu,
its still important to confirm the factory
calibration. Also note that while the output
circuitry is capable of driving loads of 600
ohms or greater, the output level will drop
slightly as the load impedance ap-
proaches 600 ohms. A 0.5dB difference in
output level can be expected as the load
impedance changes from 20k ohms to
600 ohms.
In practice, calibrating the output level
is quite simple:
1. Confirm that both the left and right
channels of the setup direct output
are terminated with their normal load
impedance.
2. Ensure that the Model 750 is con-
nected to mains power.
3. Activate the setup sections single tone
output by placing the single tone
button to the on (in) position. The voice
and multi-tone buttons should be in
their off (out) position.
4. Using an appropriate adapter cable,
connect an audio level meter directly
across the left channel setup direct
output. Ensure that your connection
does not remove the normal load! The
level meter must be a precision device
thats intended for audio usea
general-purpose voltmeter is not
adequate!
5. Carefully observing the level meter,
adjust the trim pot associated with the
left channel to give the desired output
level.
6. Repeat steps 4 and 5, this time for the
right channel.
External Record/TB Audio
Source
During installation, an external source of
audio may have been connected to serve
in lieu of the Model 750s internal micro-
phone. This audio source will be used for
voice recording by the setup section, as
well as by the talk back functions. The
external audio source needs to have a
nominal audio level of 20 to +4dBu. A
trim potentiometer, located on the back
panel directly above the record/TB exter-
nal audio input jack, adjusts the input
sensitivity of the circuitry. For correct
operation it is crucial that this trim pot be
adjusted so that the source audio level is
matched with the requirements of the
Model 750.
The procedure is a bit convoluted, but not
difficult:
1. Ensure that mains power is connected
to the Model 750.
Model 750 User Guide Issue 3, May 1997
Studio Technologies, Inc. Page 19
Model 750
2. Select the external audio source by
placing front panel configuration DIP
switch 1 to the down position.
3. Select the main bus as the VU meter
source by placing the meter source
select button to the out position.
4. Place the main output limiter/compres-
sors to the off state by placing the
button labeled LIMITER to the out
position.
5. Set the main bus master output level
controls to the unity gain position. The
front panel contains a graphic arrow
symbol and the letter U to serve as
a guide.
6. Press and hold the button labeled
TALK TO MAIN AND AUX. This con-
nects the external audio source to the
main and AUX audio buses.
7. With a normal voice signal coming
from the external audio source, adjust
the trim pot so that the VU meters
deflect to the 0 VU position on signal
peaks. You may want to enlist the help
of another person to serve as a live
audio source while you make the
adjustment. It is very important that the
external audio level reflect normal
operationif your operators or pro-
ducers love to yell and scream, set the
trim pot accordingly.
VU Meter Calibration
Two trim potentiometers allow calibration
of the VU meters normal operating level.
The range of the trim pots allow 0 on the
meters to represent an output signal level
of 2 to +10dBu. While providing a power-
ful feature, these trim pots must be ad-
justed very carefully. Be warnedmeter
calibration effects the audio performance
of the entire Model 750! From the factory,
the VU meters are calibrated so that 0 VU
represents an output level of +4dBu on
main output 1, when loaded with 100k
ohms. In certain cases it may be a good
idea to check the meter calibration, such
as when main outputs 1 and 2 are loaded
with 600 ohms, or when main output 3 is
going to serve as the primary output.
In some installations it may be desirable to
have a level different from +4dBu, such as
Figure 7. Detail of back panel showing trim
pots for VU meter calibration
Issue 3, May 1997 Model 750 User Guide
Page 20 Studio Technologies, Inc.
Model 750
0dBu, to serve as the 0 reference. While
Studio Technologies is not excited about
such a prospect, it can certainly be
achieved. There is one serious downside
to changing the nominal operating level;
the limiter/compressor threshold. The
limiter/compressors are designed to
activate at approximately 6dB above the
Model 750s internal operating level,
corresponding to an output level of
+10dBu. The threshold is not factory
calibrated and is not easily modified in the
field. If, as an example, the meters are re-
calibrated for a nominal output level of
0dBu, the limiter/compressors will now
activate at 10dB above this new nominal
level. Prior to changing the operating level,
a discussion with personnel at the factory
may be warranted. Call us for a chat!
While not difficult, meter calibration is a bit
tedious:
1. Ensure that mains power is connected
to the Model 750.
2. Place the main output limiter/compres-
sors to the off state by placing the
button labeled LIMITER to the out
position.
3. Activate the setup sections single tone
output by placing the single tone
button to the on (in) position. The voice
and multi-tone buttons should be in
their off (out) position.
4. On the setup section, place the main
bus assignment button to the on (in)
position.
5. Select the main bus as the VU meter
source by placing the meter source
button to the monitor (in) position and
selecting main stereo. This makes the
meters most accurate when they are
monitoring the main bus via the moni-
tor section. If you wish to have the
meters most accurate when they are
connected directly to the main bus, set
the meter source button to the main
(out) position.
6. Designate main output 1, 2, or, in
rare circumstances, 3 as the desired
primary output. This is the output that
the meters will be calibrated against. If
main output 3 is selected, set its output
level button to the line (in) position.
7. Confirm that the left and right channels
of the designated main output are
terminated with their normal load
impedance.
8. Using an appropriate adapter cable,
connect an audio level meter directly
across the left channel of the desig-
nated main output. Ensure that the
meter connection does not remove the
normal load. The level meter must be a
precision device thats intended for
audio usea general-purpose volt-
meter is not adequate!
9. Carefully observing the audio level
meter, set the main bus left channel
master output control so that the out-
put level matches the desired nominal
operating level. In most cases this will
be +4dBu.
10.Observing the Model 750s left channel
VU meter, adjust the left channel meter
trim pot to give 0 VU.
11.Repeat steps 8 through 10, this time
for the right channel.
Model 750 User Guide Issue 3, May 1997
Studio Technologies, Inc. Page 21
Model 750
tion DIP switches. One is shipped with
each Model 750, being attached to the
front panel by means of two screws.
Where the Model 750 is physically located,
and who will be operating the unit, will
dictate whether the security cover remains
in place or is permanently removed. In a
facility where many operators have access
to the unit, it may be best to set the con-
figuration DIP switches to match the needs
of most operators, then replace the cover.
In a facility where only a few operators
have access, such as an SNG vehicle, the
cover can stay off to allow quick access to
the features.
A brief description of the function of each
configuration DIP switch is contained on
the front panel, directly above the DIP
switches. A more detailed description is
provided here:
1. REC/TB AUDIO SOURCE: EXT
▼▼
▼▼
▼
/ INT
▲▲
▲▲
▲
This switch selects which audio source will be
used by the voice record and talk back sections.
In the down position the external audio source is
selected. In the up position the internal micro-
phone will be used.
Be careful with this switch! In most cases an
external audio source will not be connected and
the internal microphone needs to be selected for
use. Its easy to accidentally set DIP switch 1 to
the down position and have no source of record
and talk back audio.
2. REC L+R: DISABLE
▼▼
▼▼
▼
/ ENABLE
▲▲
▲▲
▲
This switch selects whether the setup sections
L+R voice segment can be recorded. In the down
position recording cannot take place. In the up
position recording can take place.
This feature can prove useful where a semi-
permanent L+R voice segment needs to be re-
corded. By placing the switch to the down
position, the L+R voice segment cannot be
changed.
Configuration DIP Switches
Studio Technologies is sorry to report that
the 8 configuration DIP switches had to be
placed on the front panel. We tried to
place them on the back panel where
theyd be hard to reach, or inside the unit
where youd never find them, but we just
couldnt find the spacejust kidding! The
8 configuration DIP switches are located
on the front panel where you can easily
get at them. These switches allow impor-
tant operating parameters to be easily
configured, and then quickly changed
on-the-fly as operating requirements
dictate. The factory default is to have all
8 switches set to the up position.
A clear plastic security cover can be
used to prevent access to the configura-
Figure 8. Detail of front panel showing
configuration DIP switches
Issue 3, May 1997 Model 750 User Guide
Page 22 Studio Technologies, Inc.
Model 750
output. Also, if the VU meter source button is set
to follow the monitor section, the AUX bus signals
will automatically be connected to the VU meters
whenever AUX interrupt is active.
8. EXT 2 INPUT LEVEL: 10
▼▼
▼▼
▼
/ +4
▲▲
▲▲
▲
This switch is used to select the nominal input
level of external monitor input 2. In the down
position the nominal input level is 10dBu. In the
up position the nominal input level is +4dBu.
3. REC L ONLY/R ONLY: DISABLE
▼▼
▼▼
▼
/ ENABLE
▲▲
▲▲
▲
This switch selects whether the setup sections
L Only and R Only voice segments can be re-
corded. In the down position recording cannot
take place. In the up position recording can take
place.
This feature can prove useful where semi-
permanent L Only and R Only voice segments
need to be recorded. By placing the switch to the
down position, the L+R voice segment cannot be
changed.
4. SINGLE TONE: 400HZ
▼▼
▼▼
▼
/ 1KHZ
▲▲
▲▲
▲
This switch selects whether the setup sections
single tone feature will generate a 400Hz or a
1kHz sine wave signal. In the down position
400Hz will be generated. In the up position 1kHz
will be generated.
5: MULTI-TONE SEQUENCE: BASIC
▼▼
▼▼
▼
/
ADVANCED
▲▲
▲▲
▲
This switch selects whether the setup sections
multi-tone feature will generate a basic or an
advanced sequence of tones. In the down position
the basic sequence will be generated. In the up
position the advanced sequence will be gener-
ated. Refer to appendices A-1 and A-2 for detailed
descriptions of the tone sequences.
6: AUX INT SPEAKER: NO
▼▼
▼▼
▼
/ YES
▲▲
▲▲
▲
This switch selects whether the AUX audio bus will
automatically be selected as the audio source for
the monitor speaker whenever an input channel or
the setup section is assigned to the AUX bus. In
the down position a signal assigned to the AUX
bus will not impact the monitor speaker output. In
the up position any signal assigned to the AUX
bus will automatically be connected to the monitor
speaker output. Also, if the VU meter source
button is set to follow the monitor section, the AUX
bus signals will automatically be connected to the
VU meters whenever AUX interrupt is active.
7. AUX INT PHONES: NO
▼▼
▼▼
▼
/ YES
▲▲
▲▲
▲
This switch selects whether the AUX audio bus will
automatically be selected as the audio source for
the headphone output whenever an input channel
or the setup section is assigned to the AUX bus.
In the down position a signal assigned to the AUX
bus will not impact the headphone output. In the
up position any signal assigned to the AUX bus
will automatically be connected to the headphone
Figure 9. Detail of front panel showing mic/
line input channel (typical of 4)
Model 750 User Guide Issue 3, May 1997
Studio Technologies, Inc. Page 23
Model 750
Operation
While the Model 750 Audio Mixer is quite
simple to operate, there are nuances to its
design that make a detailed discussion
worthwhile. While you can certainly con-
sider the Model 750 a small audio mixer,
its roots are from the much larger audio
consoles that are used in broadcast and
recording settings. Well start with a dis-
cussion of the individual sections that
make up the Model 750. Then well review
how the sections work together to become
your audio master control.
Mic/Line Inputs
Four identical input channels are provided,
each being compatible with a microphone
or line-level signal. A detailed description
of one mic/line channel follows:
Input Sensitivity
The mic/line button is used to select the
sensitivity of the input circuitry. In the mic
(out) position, the expected nominal input
level is 55 to 35dBu. In the line (in)
position, a 40dB pad is inserted into the
circuit, making the expected nominal input
level 15 to +5dBu. The input circuitry is
compatible with a wide range of signal
levels and is protected from overload. In
most cases, an input signal whose nomi-
nal level is outside the expected range
shouldnt pose a problem.
Phantom Power
The button labeled +48V is used to acti-
vate phantom power for use with con-
denser-type microphones. The +48V
power supply and related circuitry was
carefully designed to allow virtually any
microphone that requires phantom power
to operate correctly. When the button is in
the off (out) position, power is not applied
to the mic/line input connector. When the
button is in the on (in) position, and the
mic/line button is in the mic (out) position,
+48Vdc is applied to the input connector.
When the mic/line button is set for mic,
and a condenser microphone is con-
nected, changing the phantom power
on/off status may result in loud pops
being generated. As such, it is best not
to change the position of the +48V button
while its associated input is live or
on-air.
While it is a good idea to leave the +48V
button in the off position except when
phantom power is required, no damage
should occur to dynamic or ribbon micro-
phones when theyre connected to the
input with phantom voltage present.
(Wow, thats why they call it phantom
powerif a device doesnt require power
it doesnt see it!)
Low-Cut Filter
The button associated with the bent line
graphic symbol is used to select the low-
cut filter function. (If you are a stickler for
semantics, its actually a high-pass filter
function!) In the off (out) position the filter
is not placed in the audio path, leaving the
signal essentially flat. In the on (in)
position the signal is routed through the
filter circuit, removing low-frequency
energy.
The filter is useful for eliminating wind
noise, traffic rumble, and AC mains hum. It
was carefully designed to give a steep
slope (18dB/octave), with the 3dB point
at 85Hz. For most voice applications the
filter can be left activated. For wide-band-
width signals, such as music, the filter
should be used only as necessary.
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